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002100 Hydrology for the Rio Grande has been developed by the Albuquerque District, Corps of Engineers for Alamosa, by the Federal Insurance Administration for Monte vista and Del Norte, and by the Soil Conservation Service for South Fork, All agencies used recorded stream gage data for their analysis. The hydrologic data for the flood impacted reach of the Rio Grande is presented in the following table: RIO GRANDE FLOOD DISCHARGES Drainage Flood Frequency Location Area la-year 50-year 100-year 500 -year Alarnosa 1710 4,600 9,000 10,900 18,000 Monte Vista 1590 9,320 11,850 12,880 15,150 Del Norte 1320 7,701 9,879 10,734 12,608 South Fork 1164 7,360 10,620 12,230 16,530 In comparing the recorded peak discharges with the flood frequency information, it appears that the magnitude of the flood ranged from about one in a 30-year event above Del Norte to about a one in 10-year event in Alamosa. The reduction in magnitude is due in part to diversions by irrigation ditches and natural storage in the valley floodplain, There are about 50 points of diversion for irrigation ditches on the Rio Grande between South Fork and Alamosa. It has been estimated that approximately 2,500 cfs can be diverted from the river through those ditches. For floods of low magnitude, say below the 20-year return interval, these irrigation diversions have a significant impact on reducing the flood peak downstream, However. for a 100-year or greater magnitude flood, these diversions will have' a minimal impact 'on reducing flood losses. No: formal agreements for operation of the ditches for flood control purposes are known to exist between the private ditch companies and the towns. 8 ~ L , ;= i' I- I I ,~ ~~~ r c-